1. Field of the Invention
This invention relates to a light control device. More particularly, it is concerned with a light control device such as a shutter or an automatic diaphragm which performs full closure of an aperture, through which light passes, or reduction in the degree of opening of the aperture by mechanical means such as a plurality of light-intercepting blades, and especially with the structure of each of the light-intercepting blades forming such light control device.
2. Description of the Prior Arts
The light-intercepting blade used in a mechanical light control device such as a shutter or an automatic diaphragm is constructed with a plurality of blades made of metal or plastic plates. By enabling this light-intercepting blade to move at a high speed, a shorter exposure time can be obtained with improved accuracy in still cameras. On the other hand, reduction inforce to be exerted for driving the shutter of the automatic diaphragm leads to an advantage such that the film wind-up operation in the camera is made lighter, thereby improving durability of the shutter or the diaphragm. In order to reduce such drive force and moreover move the light-intercepting blade at a high speed, it is necessary that the mass of the blade itself be reduced. U.S. Pat. No. 4,401,380 of the same applicants as the present invention proposes a light-intercepting blade with its one or both surfaces having been subjected to the so-called "thinning" process, by which thickness of a portion of the blade other than the marginal portion thereof is reduced.
However, the light-intercepting blade manufactured in this manner has its component blades usually formed of thin plates, and such process of making the thickness of the individual blade much thinner in part would unavoidably result in decreased blade strength. Particularly, at the termination of movement of the shutter blades, brake is applied in a very short range, which makes the impact applied to the blades themselves to be very great. In the case of a square focal plane shutter, wherein each of the forward blade group and the rearward blade group comprises several blades, brake is applied not by directly receiving the blades to stop them, but often by stopping the blade driving member (blade arm), hence the largest impact is applied to the blades at the root of the blades (the neighborhood of the joined portion between the blade arm and the blades) where the inertia of the entire blades acts, and flexure or bending may be caused in that portion.
Partial thinning of the individual shutter blades is effective in weight reduction of the blades, while it involves the disadvantages as mentioned above. Therefore, simple reduction in weight of the blades does not always insure higher moving speed of the blades.
As a way of solving the conventional problems as mentioned in the foregoing, the present inventor proposed an improvement as described in U.S. patent application Ser. No. 460,925 filed on Jan. 25, 1983, by which it has become possible to maintain sufficient strength at the root of the light-intercepting blades. After this improvement made, he continued further studies on the manufacture of more ideal light-intercepting blade, in the course of which he discovered that, with reduction in weight, a problem of the blade strength occurs even at or near the free end of the light-intercepting blade away from the joined portion between the blade and the blade arm, and that this new problem could not be solved by the abovementioned improved technique.